Olefins are mainly obtained by cracking naphtha or natural gas, but their purification requires a rather complicated and costly process because of the co-production of small amounts of acetylenic hydrocarbons with similar boiling points. Acetylenic compounds present in olefins can serve as catalyst poisons in the olefin polymerization process and also lower the product quality of the resulting polyolefins. Furthermore, the acetylenic compounds are sometimes converted into solids, blocking the fluid stream and even leading to explosion. Therefore, it is crucial to eliminate even the small amount of acetylenic compound contained in olefin.
The removal of acetylenes present in olefins may be commercially accomplished through a partial hydrogenation of acetylenes into olefins using a noble metal catalyst like supported Pd. However, olefins produced from the hydrogenation of acetylenes, as well as feed olefins, may undergo an over-hydrogenation reaction to produce paraffins, thus resulting in the loss of olefins. Furthermore, there may be severe catalyst poisoning resulting from the carbon deposition, and additional steps for regenerating the catalysts are required, as described in U.S. Pat. Nos. 3,755,448 and 3,792,981.
Solvent extraction using organic solvents, such as DMF (N,N-dimethylformamide) or NMP (N-methylpyrrolidinone), is another commercial process for separating acetylenes from the cracking process. Acetylenes thus obtained as the unavoidable by-products from the cracking process are the major source of acetylenes used for various organic syntheses. However, the extraction process is also technically disadvantageous and uneconomical, particularly due to the significant loss of solvent after multiple operations and to the low selectivity of acetylenes over olefins.
In some cases, the separation of unsaturated compounds from saturated hydrocarbon mixtures may also be carried out by energy intensive low temperature distillations.
With respect to solid adsorption, U.S. Pat. Nos. 4,019,879 and 4,034,065 teach methods for adsorbing and removing unsaturated compounds such as CO by using molecular sieves; however, these methods are not only limiting in terms of the adsorption capacity but also require a high temperature and high vacuum pressure for the stripping step.
To circumvent the problems encountered in conventional hydrogenation, solvent extraction, and low temperature distillation, separation through reversible π-complexation using a Ag(I) or Cu(I) salt has been discussed extensively.
Thus, U.S. Pat. No. 4,717,398 provides a method for removing unsaturated compounds by pressure swing with an adsorbent comprising faujasite zeolite substituted with Cu(I). In addition, Japanese Patent Laid-Open Publication No. 1986-50929 discloses a method for removing acetylenic compounds by using an adsorbent comprising alumina, silica or active carbon loaded with Cu(I) or Ag(I) compounds; however, this method is problematic because the acetylenic compounds are likely to react with Cu(I) or Ag(I) to produce unstable copper- or silver-acetylides. Therefore, there is a demand for the development of an improved adsorbent that can prevent production of these materials.
German Patent No. 2059794 teaches a method of removing unsaturated compounds comprising acetylenic compounds by using a liquid absorbent mainly consisting of Cu(I) compounds and alkanolamines such as monoethanolamines; however, this method has the drawback of requiring an additional purification device due to the contamination of the final product by alkanolamines and co-absorption of olefin. Safarik D J, et al., Ind. Eng. Chem. Res. Vol 37, No. 7, 2571-2581, 1998 discloses a method of separating unsaturated compounds from paraffin by using a Cu(I) or Ag(I) compound solution which is subject to a reversible reaction with olefin and acetylene; however, this method is also problematic in that it requires a complicated regeneration process because of the low stability of the adsorbent.
As for membrane separation, U.S. Pat. No. 3,758,603 discloses a method of separating unsaturated compounds from saturated compounds by using liquid membranes prepared by loading silver salts with microporous membranes. According to the above method, however, salts for facilitated transport are likely to be washed away and solvents are easily volatilized, thereby making it difficult to maintain separation efficiency for a long time. U.S. Pat. No. 4,318,714 describes a method of using ion exchange resin membranes in which cations are substituted with anions, in order to prevent the silver ions from being washed away. In such cases where fixed liquid membranes are used, since facilitated transport only takes place when moisture exists, it is a nuisance to have to maintain the moisture level inside the membranes constant and remove the moisture again after the separation. In addition, not only is the above method impractical since it requires a membrane thickness of from 100 to 500 μm but the separation efficiency is also low.
Therefore, in addition to the formation of explosive acetylides, the unsatisfactory selectivity of acetylenes over olefins due to the similar bonding strength of acetylenes and olefins to Ag(I) or Cu(I) salt has always been problematic. Hence, there has been high interest in the field to search for efficient sorbents that can selectively and reversibly interact with acetylenes.